The number of automobiles has been increasing drastically. The amount of gas discharged from the internal combustion engines has also increased drastically in proportion to the increase in the number of automobiles. The various substances included in the exhaust gas of a diesel engine have caused pollution, which has a severe impact on the world environment. Further, there have been recent reports on research results stating that fine particulates included in the exhaust gas cause allergic disorders and a decrease in the number of sperms. Accordingly, the removal of the particulates from the exhaust gas is a problem that must be immediately coped with by mankind.
Under such circumstances, various types of exhaust gas purification apparatuses have been proposed. An exhaust gas purification apparatus includes a plurality of filters that eliminate particulates discharged from internal combustion engines, such as diesel engines. When a filter is used for a long time, particulates are deposited in the filter. The deposited filter increases the load on the engine. Thus, the particulates must be eliminated.
Presently, when the particulates on a filter approach a maximum collection amount, a switch valve switches the filter through which exhaust gas flows. The filter through which the exhaust gas no longer flows is heated by an electric heater until it reaches a temperature at which the particulates are ignited. The filer is regenerated by burning and removing the particulates.
However, in the conventional purification apparatus, the electric heater heats the filter even if the initial temperature of the filter that is to be regenerated is lower than the temperature of the engine exhaust gas. This prolongs the filter regeneration time. For example, when the diesel engine is running continuously, the filter temperature is about the same as the exhaust gas temperature. However, when the diesel engine is stopped over a long period, the filter temperature is low, and the increase rate of the filter temperature is low. Thus, much time is required for the particulates to reach the ignitable temperature. This increases the regenerating time. In addition, the prolonged regenerating time lengthens the activation time of the electric heater, increases power consumption, and lowers the durability of the electric heater.